Replace server actions with API routes and add SWR data-fetching hooks

- Convert discover, stats, status, and system-health server actions to
  proper API route handlers under `app/api/`; delete `lib/actions/explore.ts`,
  `lib/actions/settings.ts`, and `lib/actions/setup.ts`
- Add `use-discover`, `use-stats`, and `use-system-health` SWR hooks
  that call the new routes; update `command-palette`, `title-card`,
  `update-toast`, and `stats-display` to consume them
- Lift auth centering wrapper from individual login/register pages into
  `(auth)/layout.tsx`; switch both pages from `auth.api.getSession` to
  the cached `getSession()` helper
- Relocate setup wizard from `app/(auth)/setup/` to `app/setup/` (outside
  auth group) with dedicated `copy-button` and `refresh-button` client
  components
- Move `not-found.tsx` and `error.tsx` to app root so they apply
  globally instead of only within the pages route group

proxy.ts

@@ -1,7 +1,7 @@
 import { getSessionCookie } from "better-auth/cookies";
 import { type NextRequest, NextResponse } from "next/server";
 
-const authRoutes = new Set(["/login", "/register", "/setup"]);
+const authRoutes = new Set(["/login", "/register"]);
 
 export function proxy(request: NextRequest) {
   const { pathname } = request.nextUrl;